Toenail fungus, or onychomycosis, is an infection underneath the surface of the nail caused by fungi. When the tiny organisms take hold, the nail often becomes darker and smells foul. Debris may collect beneath the nail plate, white marks frequently appear on the nail plate, and the infection is capable of spreading to other toenails, the skin, or even the fingernails. If ignored, the infection can spread and possibly impair your ability to work or even walk. The resulting thicker nails are difficult to trim and make walking painful when wearing shoes. Onychomycosis can also be accompanied by a secondary bacterial or yeast infection in or about the nail plate.